1. Field of the Invention
The present invention relates to an optical module and a method of manufacturing an optical module.
2. Description of the Related Art
A can-type optical module used in optical communication generally includes an electrically-grounded stem and a lead terminal that passes through the stem and is insulated from the stem. The stem and a cap mounted to the stem form a housing for accommodating an optical semiconductor device having a transmitting function and a receiving function. The lead terminal and the stem form a coaxial line. One endportion of the lead terminal is connected to the optical semiconductor device. Another end portion of the lead terminal is connected to a drive device configured to output a modulated electrical signal or to an integrated chip (IC) configured to amplify a signal photo-electrically converted by a light receiving element, via a wiring substrate such as a flexible printed circuit (FPC) having a signal line and a ground formed along the signal line. In order to simplify a description, a can-type optical module having a transmitting function is hereinafter used as an example.
Increases in communication speed are leading to an increase in the frequency of the modulated electrical signal output from the drive device. Due to the increase in the frequency of the electrical signal, reflection of the electrical signal tends to occur in a transmission line at locations at which there are mismatches in the characteristic impedance. Characteristic impedance mismatches tend to occur between, for example, the coaxial line, which is formed by the lead terminal and the stem, and the wiring substrate such as an FPC. Further, characteristic impedance mismatches may also occur between the coaxial line, which is formed by the lead terminal and the stem, and a portion of the lead terminal that is protruding from the stem. When a reflection occurs at a location at which there is a characteristic impedance mismatch, the reflection wave may be further reflected by the drive device, causing multiple reflections to occur in the transmission line. When such reflections occur, distortion occurs in the original modulated electrical signal, causing the waveform quality of the optical signal to deteriorate.
In order to suppress to a minimum the effects of characteristic impedance mismatches on the optical waveform at a connecting portion between the lead terminal and the FPC at which characteristic impedance mismatches tend to occur, Japanese Patent Application Laid-open No. 2006-080418 and Japanese Patent Application Laid-open No. 2009-295717 propose structures for suppressing characteristic impedance mismatches.
Specifically, Japanese Patent Application Laid-open No. 2006-080418 discloses a structure that uses a stem having a trench formed in a welded portion of a ground pin in order to increase the close contact properties of the connecting portion between the FPC and the stem, thereby preventing an increase in the inductance at the lead terminal portion. In Japanese Patent Application Laid-open No. 2009-295717, the wiring pattern of the FPC is processed so that changes in the impedance between the FPC and the coaxial line formed by the lead terminal and the stem are lessened, to thereby ensure sufficient optical waveform properties.